Elastic time division multiplex system



April 10, 1951 c. B. H. FELDMAN ELASTIC TIME DIVISION MULTIPLEX SYSTEM 5 Sheets-Sheet 2 Filed Feb. 10, 1949 F/G. A

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A T TOR/VE V Patented Apr. 10, 1951 UNITED STATES PATENT OFFICE ELASTIC TIME DIVISION MULTIPLEX SYSTEM Carl B. H. Feldman, Summit, N. J., assignor to Bell Telephone Laboratories,

Incorporated,

8 Claims. l

This invention relates to multiplex communication, and more particularly to the transmission of telephone calls on a time division multiplex basis.

The principal object of the invention is to provide for the handling of calls originating with a number of independent subscribers by a lesser number of channel facilities, thus reducing the fraction of the time during which a transmission channel would otherwise be idle, and so enabling economies to be effected in the provision of transmission channel facilities.

A more specific object is to provide for the handling of the simultaneously active few of a number of independent and potentially active callers on a time assignment basis.

Another object is to interpolate the speech of talkers of lower urgency ratings in the pauses between spurts of speech of talkers having higher urgency ratings.

These and other objects are attained, in accordance with the invention, by the provision of a sequence of channel pulses whose locations on the time scale are not fixed, but variable as a function of the distribution of activity among the several talkers. When a particular talker is for the moment active, the :channel pulse assigned to him is given a busy character which introduces a time delay before the accurrence of the next channel pulse in order; and in the interval thus provided, the instantaneous amplitude of that talkers speech is translated into a permutation code group of pulses, and so transmitted. The same sequence of events takes place for each active talker in turn, until the available channels have been filled. When, however, any one of the talkers is instantaneously silent, his channel pulse is given an idle character, the corresponding zero amplitude code pulse group is suppressed, and the time interval which would otherwise be assigned to it is skipped. This idle channel pulse is immediately followed by a channel pulse for the next talker in order, and so on, each idle channel pulse being immediately followed by another channel pulse, each busy channel pulse being followed by a permutation code pulse group which is in turn followed by the next channel pulse. At the receiver terminal, the character of each channel pulse, busy or idle as the case may be, sets in operation apparatus which either decodes the ensuing group of permutation code pulses or skips the time interval in which the permutation code pulses representing the zero speech amplitude of the silent talker would otherwise occur.

Among the features of the invention are the apparatus which introduces the time delay between the channel pulse of each talker and of the next one in order, which is required, when the former` is active, to provide time for the coding of his speech sample. In a preferred arrangement, a plurality of similar impulse coils are provided one for each talker, each with its saturable ferromagnetic core. They are supplied with current of sawtooth wave form by way of static time delay circuits so that the magnetic condition under which the pulse arises occurs for the several coils in time sequence, each being delayed with respect to its neighbor of lower number by a single pulse period. In addition, each coil'is provided with a control bias winding, and these are connected in series. Activity of anytalker gives rise toA a current of standard magnitude which flows through the control bias windings of all of the coils having higher numbers. The magnetizing force of this current opposes that of the sawtooth current,

and so serves in eiect to delay the pulsinginstants of all such coils having higher numbers by a time sufficient to permit the interpolation of a permutation code group of pulses corresponding to the speech amplitude sample of the active talker. The process is repeated for each active talker in turn until the time of thetransmission cycle has been fully utilized. Talkers who happen to be active at this instant are skipped, and must await their turn in the following operation cycle. Such a situation rarely occurs when the system is properly designed in respect to number of talkers and number of channel facilities. Similar impulse coils, provided With similar bias windings similarly energized, are provided for the allotment of appropriate time intervals to the decoding of incoming signals of the active talkers, while economizing the corresponding times in the case of in' active talkers.

Because of the manner in which the transmission channels are shifted on the time scale in accordance with the activity distribution among the talkers, the present system may be termed an elastic time division Vmultiplex system.

The invention will be fully apprehended from the following detailed description of an embodiment which, for illustrative purposes, is taken as a system for transmitting the telephone calls originating with eight potential talkers and destined for eight listeners. Of these eight potential talkers, the speech of any three instantaneously active talkers is transmitted on a time division basis. Again for purposes of illustration, speech amplitudes are transmitted in the form of six-digit binary code pulse groups. Each such code pulse group is preceded by a busy channel pulse and is followed by a blank pulse position which is provided to allow the coding apparatus to recover and be in readiness for the next coding operation. Thus each operation cycle or frame period is made up as follows:

Channel pulse periods 8 3 code pulse groups, six digits 18 3 coder recovery periods 3 1 frame synchronizing pulse- 1 Pulse periods per frame 3i) apparatus of Fig. 1;

Fig. 5 is a schematic diagram, partly in block form, of elastic time division multiplex receiver terminal apparatus;

Fig. 6 shows groups of wave form diagrams of assistance in explaining the operation of the apparatus of Fig. 5.

Referring now to the drawings, and in particular to Fig. 1, several talkers A to I-I, inclusive, actuate an equal number of telephone transmitters I I to 1 8 at will and at random. Each ofthese transmitter instruments,y when so actuated, furnishes speech energy by way of a transformer 2 I to 2 8 to a sampling clamp 3 I to 3 8. These clamps are all operated simultaneously, once in each time division cycle or frame period, by application of the pulse output of coil 4 0 to its control terminal designated by an arrow. When so actuated, these clamps serve to place on the storage condensers 5 I to 5 8 electric charges which are proportional to the instantaneous amplitudes of the speech waves of the respective talkers.

The sequence Jof events required for operation of the whole system is controlled by recurring -sawtooth current waves.

These current waves may be generated in any desired fashion but a convenient one comprises the application of a sawtooth voltage wave, of the form shown in Fig. 2A, derived from a timing generator 6 to progressive phasing apparatus. The latter may comprise a tandem-connected'sequence of delay `well-known manner enables the delay between the voltages appearing at successive tapping points to be adjusted with precision to one pulse period. These various tapping point voltages are applied to the control grids of a like number of tubes, for example triodes 3 8 to 8 8 each of which has a resistor 9 9 to 9 8 of high ohmic value connected in its cathode circuit to provide substantial negative feedback. As is well known, these connections result in the cathode voltage and the anode current following the applied control grid voltage, so that the anode currents of the several tubes have substantially the same sawtooth wave form as the voltage of the source 6, each being delayed with respect to the anode current of the tube above it by the delay contributed by one of the delay devices l. These wave forms are shown in Fig. 2B.

In the anode circuit of each of these tubes 8, there is connected an impulse coil 4. A second direct-current bias winding I0 is wound on the same core I4 as the impulse coil 4 and carries a fixed bias current derived from a battery II and adjustable in magnitude by a variable resistance I2. A third winding i3 2 to I3-8 is wound on the same core I4 in the case of all of the triodes except the rst two. These Ythird windings serve as control bias windings, each one carrying a variable bias current which depends on the distribution of activity among the several talkers in the manner to be described below. These third control bias windings I3 are all connected in series and by way of a battery to ground.

Disregarding, for the present, the action oi the third winding I3, the currents in the individual direct-current bias windings il) are so adjusted that, when the discharge currents ei the triodes 8 have their minimum values, i. e., at instants corresponding to the most negative part of the sawtooth voltage wave of the source i3, the net magnetizing force in each case, due to the combined action of the impulse winding II and the bias winding Iii and the currents liowing in them is slightly negative with respect to the value at which the permeability of the core rises sharply. This is illustrated by the lower portions of the variously delayed sawtooth waves of magnetizing force of Fig. 2.

Now when the current in the impulse winding of coil 4 (9 commences to rise, the magnetizing force on the core due to this current balances the magnetizing force due to the current in the bias winding Ill- 0 so that the net magnetizing force on the core is justsuilcient to bring the magnetic condition into the high permeability region. At this instant a pulse (Fig. 2C) occurs at the output terminals of the first winding Q B and this pulse is applied by way of the conductor I5 to all of the two-way clamps 3 simultaneously. It serves to clamp the instantaneous speech amplitudes of the several talkers A to H, inclusive, onto the Yseveral storage condensers 5 I to 5 8, inclusive, and so to store samples of their speech for future use. Ihis same pulse is also delivered by way ot a conductor I6 to the output terminals or" the transmitter apparatus for transmission to a receiver station where it serves to establish synchronism between the events taking place there and those taking place at the transmitter. A delay device il and an ampliier I8 are interposed in this path, the first to balance miscellaneous delays originating in other components of the transmitter apparatus the second to increase the amplitude of thisY pulse as compared with message code pulses, so that it may be sorted, at the receiver, on an lamplitude basis.

5, After a time interval equal to a single pulse period, hereinafter designated t and determined by the design of the delay device 1-l, a delayed version (curve l of Fig. 2B) of the sawtooth current wave flows through the impulse winding of the second coil 4-l and by actions identical to those discussed above, a pulse appears at the output terminal of this impulse winding which is delayed by the time t with respect to the coil 4 0 pulse. Similarly with respect to the remaining coils of the groups 4 0 to 4-8; in the absence of current in the third windings i3 of the several coils, the output terminals of the several impulse windings deliver pulses in regular time succession.

After the completion of the rise of current in each impulse winding, it commences to fall. In the course of this fall, the magnetizing forces in the several cores pass through the pulsing values and output pulses will again occur in sequence. However, these pulses are of opposite polarity and have no eifect on the system. Such a negative pulse is shown for each of the coils 4 in the lower part of Fig. 2.

After the completion ef the fall of current in the last coil 4-8, the cycle of operations is repeated.

Coming now to the elastic feature of the system, which involves effectively delaying all of the channel pulses for channels following an active talker, the following description explains the operating principle.

When the coil 4-l pulse occurs it is applied by way of a blocking condenser 2|l to a coincidence gate I 22-1 which operates a single trip multivibrator 23-l provided it is enabled by the existence of a standard voltage which signifies activity of talker A, and not otherwise. 'I'he gate I 22-l may comprise, for example a pentode, to whose suppressor gate one input is applied, While the other input is applied to its control grid. This enabling voltage may be derived from any suitable speech sample detector 2,4-l which may be an electronic relay which responds by delivering a voltage of standard magnitude and sign when the speech sample stored on the storage condenser -l has a value other than zero, thus signifying that talker A is presently active. By a value other than zero is meant a value equal to or greater than the smallest speech amplitude to be recognized as such and transmitted in code. Since this smallest speech amplitude which it is desirable to transmit may be smaller than the amplitude of noise of low audibility (such as low frequency hum present in the talkers line) it may be desirable to supplement the sample detector 24-l which operates on each speech sample, with a speech detector 25-I, tuned to speech frequencies and having the ability, by frequency selection, to distinguish between continuous speech and these noises of low audibility. The relay of the speech detector connects the storage condenser 5-l to ground during the pauses between spurts of speech and insures that noise will not be mistaken for genuine talker activity. If talker A is inactive, the output of the sample detector 24-l remains Zero and the coincidence gate ZZ-l remains disabled, independent of the presence of noise. In this pulse period, following the synchronizing pulse, no pulse is transmitted to the receiver if talker A is inactive.

The speech sample detector may comprise a pair of slicer circuits connected back to `back-and provision for inverting the output of one of them.

Fig. 3 shows a suitable circuit arrangement in which each of the slicers comprises a two-tube multivibrator, the grid of the right-hand tube 30b, 3i-b, being supplied with an adjustable bias potential, for example by way of a potentiometer connected between ground and a suitable steady potential, negative (-C) or positive (-l-C) as the case may be. Thus the bias on the grid of the right-hand tube 30-b of the upper multivibrator may be adjusted by a potentiometer 32 to a negative value such that, in the absence of input signals, the stable condition of this multivibrator is that in which the left-hand tube is conducting and the right-hand tube is non-conducting, whilein the case of the lower multivibrator a positive bias voltage is applied, by` way of the potentiometer 33, to the grid of the right-hand tube 3 l-b, so that in the absence of the signal iV, the right-hand tube is conducting while the. left-hand tube is non-conducting. The output is taken from the normally non-conducting tube in each case, i. e., from the anode ofthe right-hand tube BIJ-b of the upper multivibrator and from the anode of the left-hand tube 3|-a of the lower multivibrator. These outputs are individually applied to the control grids of buifer amplifiers 34, 35 whose anodes are connected together. With this network, when a positive voltage is applied to the input terminal 36, it has no effect on the upper multivibrator, but if it exceeds the preassigned threshold set by the tap on the potentiometer 33, it causes the left-hand tube 3l-a of the lower multivibrator to conduct, the righthand tube 3|-b becoming non-conductive. HThis applies a negative voltage increment to the grid of a lower buffer tube 35 and cuts this tube oiT, thus causing a positive voltage of standard magnitude to appear at the output terminal 31. Similarly, When a negative speech voltage is applied to the input terminal 36 of the device, it has no effect on the lower multivibrator, but when it exceeds a preassigned threshold determined by the setting of the upper potentiometer 32, it causes the lefthand tube .3U-a of the upper multivibrator to be rendered non-conductive, and the right-hand tube 30-b to conduct. This applies a negative voltage increment to the gridof the upper buffer tube 34 and cuts it off, thus applying a positive voltage of standard magnitude to the output terminal 31 of the device.

In the case of each of the multivibrators, the grid to which the control bias is applied is also supplied with a bias-equalizing network which may comprise a diode shunted by a high resistor. This serves to make the grid bias of the righthand tube of each of the multivibrators substantially independent of signal frequency conditions.

By adjustment of the magnitude of the intertube coupling condensers in each of the multivibrators and of their associated resistors, their time constants are preferably made much longer than the period of the lower speech frequency. The time constants of the alternating current coupling to tubes 34 and 35 as Well as the time constants of the alternating current coupling to preparation for' the receipt of a new value of the speech voltage.

The sequence of events throughout a ramewill be described in connection with an example, depicted in the curves of Fig. 4, in which talkers B, E and F are active, lthe others being inactive.

The next event to occur, inasmuch as talker A has been described as being inactive so that his coincidence gate I 22 l and his single trip multivibrator 23 l are not enabled, is the pulse of coil 4 2, which occurs in the next pulse period following the pulse of coil 4 I. It is applied to the second` coincidence gate I 22 2 to which is also applied the output of talker Bs speech saniple detector 24 2. Under the assumed conditions, talker' B is active so that the relay ofhis tuned speech detector -2 is opened and the output of his speech sample 4detector 24-21 has its standard enabling value and the coincidence of this output with the pulse of coil 4 2 serve to enable the gate I 22 2 and so to operate the single' trip multivibrator 2Il2r which then delivers a dat topped pulse to a gate II -2. This gate may, like the one described above, comprise a pentode,

to the control grid of which the voltage of the storage condenser 5 2 is applied while the output of the single trip multivibrator 23-2 is applied to its'suppressor grid. As is well known, with such a circuit, the charge on the storage condenser 5 2 is effectively gated to an output conductor 4l when and only when the gate 4 2 is opened by the output of the single trip multivibrator 23 2. The' speech sample of talker B is thus applied to an amplifier 42 and then to a coding device 43 for as long a time as the output pulse of the single trip multivibrator 23-2 continues. This time is adjusted to be lt as shown in curves E of Fig. 4.

In order to allow time for the production and transmission ci the code'pulse group representing the speech sample of talker B, the pulses of talkers C through H, inclusive, are now all delayed by seven pulse periods in the following manner. The output of talker Bs speech sample detector 2i 2 is applied to the grid oi a tric` de`45 2iwhich' is normally held below cut-ofi by a bias battery st lThe output of the speech `sample detector 24-2 raises the grid of this triode 45-2 above cut-ofi by a standard amount. A standard current then flows through the discharge path of this triode andV through the control bias windings of all of the coils |3 3 to [3 8, inclusive, in series. By making the resistance valuesv` of the anode resistors 4'! of each of the triodes 45` high in'v comparison with the' resistances of the contr'olbias'windings i3,- itis possible to arrangethat the current owing through these windings by the application of the speech sample detector voltage to the grid of any'of the triodes 45 shall be oi standard magnitude and independentof the number of coils lthrough which it flows. In the? assumed condition, the currentV flows under thev inuence of a battery 48 through' the discharge path of the triode -2, through itsariode resistor 41-2 and through the bias windings' of coils`I3-3 to 13 3, inclusive, whereas hadtalkei` A" been active, the same current would have ilowed through the discharge path of the triode 45-I and through its anode resistor'l-l and through coils 13 2 to {3 8, inclusive.

In'accordance with the invention, the magnitude of the current thusowing throughthese ccilsis so adjusted asrto bias the-'magnetizing force in all of their cores 'by an amount'suchtl'ia't, in the'cour's'e of the rise* of' the voltagewaveillig 2) of the saw-tooth generator E, the pulse condition will not be reached untilafter a delay of 7t; This condition is illustrated in curves A of Fig'. 4, wherein the pulse from coil 4 3 occurs at the 10th puise position of the frame instead of at the third pulse position where it is shown in Fig. 2.

Under the assumed conditions, talkers C and D are inactive, so that their speech sample detectors 24 3 and 24 4 record no stored condenser voltages other than 0, their single trip multivibrators 23 3 and 24 4 remainr disabled and their tubes 45 3 and 45 4 remain cut on'. rl'hus the zero value speech samples are not applied to the coding device 43 and the pulses derived from the impulse coils 4 3 and 4 4, respectively, take place in rapid succession, spaced only by the pulse period time interval t. However, talker E is active, so that when the pulse is' generated by the impulse coil 4 5, his speech sample detector 24-5 is delivering an output, and the coincidence gate I 22 5 and the associated single trip multivibrator 23-5 are therefore enabled and the latter opens the gate II 4(1 5 admitting the voltage oi' the storage condenser 5 5 to the coder v33. At the same time the output of the speech sample detector 24-5 is applied to the grid of the triode 45-5 so that standard current ilows from the battery 48 through the bias windings (3 6 to |3 8. This current is in addition to the current already flowing through coils l3-3 to lli-3, inclusive, and thus further biases the magnetizing force on the cores of these coils by an amount such that the pulse derived from the coil 4 5 is again delayed by another timel interval of "lt, This is illustrated inv Fig. Il' where the coil 4 5 pulse occurs in the twentieth pulseA position instead of in the sixth where, as shown in Fig. 2, it would have occurred had it not been delayed by fourteen pulse positions, i. e., by a total delay of Tt, clueV to activity of talkers B and E.' By a sequence of operations similar to that described above, the activity of talker F results in Va standard voltage output of his speech detector 24 6 which permits the coil 4 6 pulse to open the gate I 22' 6 thus admitting the sample of the talker Fs speech to the coder 43' and at the same time providing a third increment of bias' currentthrough the tubesV 45-1 and 45' 8 and the control bias windings of coils |3 l and {3 8'. The events described above have provided a time interval of 'ltV immediately following thelcoil 4 2 pulse, another time interval of 'lt immediately following the coil 4 5 pulse and a thirdvtime interval of 'it immediately following the coil 4 6 pulse; While the three intervals, of lt each, have been discussed in succession, it will be recalled that they all result from events occurring within the first t period following the samplingpulse of coil 4 8. These time intervals are employed in accordance with the invention for carrying out the operationsof coding and transmitting the speech amplitude samples of talkers, B, E and F, respectively, which, by hypothesis were active at the instants at which the several talkers transmitters were clamped to the storage condensers. The coder 43 may comprise a modification of theone shown and described in two articles published in the Bell System Technical Journal for January, 1948, volume 27, pages 1 and 44. A In brief it comprises an electron tube having an electron gun, a target anode and a coding mask 50 interposed in the path of the beam between the gun andthe target. kThe coding mask-isfprovidedw'ith perforations `or apertures" arrangedin rowsandcolunns in accordance with thedigit's' of the binary code. For a six-digit code as contemplated in the present illustration, the apertures are arranged in six vertical columns and in sixty-three horizontal rows. To avoid undue detail in the drawing, the mask is shown as arranged for a iive-digit code; i. e., with the coding apertures arranged in ve Vertical columns and thirty-one horizontal rows. One pair of deflecting elements, for example, plates I deflect the beam 52 fromits rest position on a level with the center of the coding mask to one aperture row or another in accordance with the magnitude of a speech sample to rbe coded. Another pair of deflecting elements 53 then sweep the beam across all of the apertures cfrthis particular row, giving rise to a sequence of pulses in the output circuit 54 which constitutes a per mutation code pulse group in accordance with the six-digit binary code.

In accordance with the present invention, the

coding mask is provided with certain additional features. Ln the first place, there is provided a single large aperture 55 which extends throughout the full length of the mask from its rst aperture row to the last and disposed at that side .of the mask at which the cathode beam sweep 'the sweep of the beam across the various rows of .apertures takes place in an aperiodic fashion in dependence on the activity distribution among the several talkers. To this endthe coding mask 5i) is provided to the left of the channel pulse aperture 55 with two ribbon-like electrodes 55 which, together, extend throughout the full height oi the coding mask 50 except for a bare central portion. These two electrodes are connected together and to the input terminals of a trigger circuit 5l whose output pulse operates an aperiodic coding sweep generator 55. IThe trigger circuit may be of any desired type, a suitable one being a single trip multivibrator connected to be tripped by application of negative pulses. Deflection of the cathode beam 52 upward or downward in either directionaway from its central rest position causes it to impinge on the ribbon electrode 5B and deliver a negative voltage to the trigger circuit 5l whose output sets oiT the aperiodic sweep generator 58. The latter may cornprise a resistor, a condenser, a source of voltage and a Vdischarge tube interconnected in wellknown fashion to charge the condenser relatively` slowly to a preassigned voltage and then discharge-it rapidly. As is well known, by suitable choice` of the magnitudes of the condenser and the resistor, the condenser voltage may be caused to rise substantially linearly with time for a certain fraction of its charging cycle. 'Ihis linearly rising voltage is now applied to an ampliiier :EIB which feeds the horizontaldefiecting plates 53 of the coding tube and so causes the cathode beam 52 to sweep first across the channel pulse aperture and then across the following vcode pulse apertures, of a particular row. The

sensitivity of the ampliner 59 is so adjusted that the Asweep across the coding apertures is completed after a period of 7i, that is seven. pulse periods, the iirst being .that associated with the .channel pulse and the remaining sig with the I l0 code pulse group. Thereafter the sweep is returned to its initial position by self-restoration of the multivibrator 58 to its original quiescent condition. As appears in curves C of Figz 3, a time interval t following the six-digit code is allowed for recovery prior to the next coding operation. The sweep and recovery are shown in curves F of Fig. 4. f

In the course of its sweep across the apertures of the coding mask 5E), the beam may be stabilized aginst wander and so restricted to movement along a particular aperture row by the use of a stabilizing grid as described in the aforementioned article published in the Bell System Technical Journal and as claimed in an application of L. A. Meacham, Serial No. 766,211, filed August 5,1947, and issued on June 21, 1949, as Patent 2,473,691. Throughout the horizontal or lateral sweep of the beam across the apertures of a particular row, the beam has been localized on the correct row by being deflected thereto by the application .of the signal sample voltage being codedr In particular, and in the case assumed, the nrst signal sample of the frame to be coded is that of talker B whichl has been stored on the storage condenser 5 2 and admitted to the Vertical deflecting plates of the coding tube by way of gate ,Il 40-2. At the termination of the codingtube beam sweep, the beam is restored to its rest position as above described by return of the single trip multivibrator 58 of the triggered coding sweep circuit to its original quiescent condition and at the same time the signal sample of talker This gate is brator 23-2, which originally opened this gate upon the occurrence of the pulse in coil 1 2 likewise returned to its original condition-.after the lapse of seven pulse periods, as indicated in curves E of Fig. 4. Thus the beam, after ccmpletion of the coding of the rst signal sample, returns to its rest position in the. slot between the two ribbon-like electrodes 56. YTherefore it remains for two pulse periods, during which the pulses of coils 4 3 and Il occur but fail to enable the single trip multivibrators 23-3 and M t because, by hypothesis, the speech sample detectors 2d 3 and M fl associated with talkers C and D give no output. Upon the occurrenceof the pulse in coil 5, gate I 'Z2-5 is opened, ad'- r'nitting to the vertical denection ampliiier i2 of the coding tube a voltage proportional to talker Es speech sample which is stored on condenser coding mask 5i) giving rise in the output circuit of the coding device to a channel pulse due to passage of the beam through the left-hand channel pulse aperture, immediately followed by a binary code pulse group representative of talker Es signal sample amplitude. The beam then returns tor its initial position whereupon the events are repeated with respect to the talkerF, being initiated by the occurrence of the initiating pulse in coil 4 6.

After completion of the coding of the speech sample of the talker F, pulses occur in coils 4-7 and 4-8 in rapid succession but they are of no eiect because talkers G and I-I are inactive so that their speech sample detectors 2li-l and 24--8 give no output to complete the enablement of their single trip multi-vibrators 23-1 and 23-8.

As a result of the events described above there has been created on an output conductor Eil a sequence of pulses of which each group comprises a channel pulse followed by a binary code sixpulse group and one recovery period, in the case of active talkers, while for inactive talkers only blank channel pulse positions are created.

At the conclusion of the events described above the pulse of coil 4-8 recurs and a new and different distribution of channel activity may be encountered. The magnetizing force in some of the coils 4--2 to rl--B may be restored to the ccndition shown in Fig. 2, having been in the oondition shown in A of Fig. 4. If the magnetizing force in some of the coils which had been displaced negatively in the preceding frame crosses zero (becomes positive) when the pulse of coil 4--0 occurs, premature positive pulses will appear in the corresponding coil 4 winding and will produce unwanted events at variance with the principles as described. A preferred way of insuring that this does not occur is to design all of the sweeps to 'oe negative at the time of occurrence of the coil 4-il pulse as shown in Fig. 2. This requires that the ilat portion shown on the negative side of zero in Fig. 2 must persist for at least 8 pulse periods as shown.

In order to understand the principles underlying the design of the sweeps oi Fig. 2, it will be desirable to generalize the specific case described in the foregoing, as follows:

Let N stand for the total number of potential talkers, n for the number of active talkers or code pulse groups provided for, and d for the number of digits in the code. Then, in a system embodying the principles described above, each pulse period is made up, in general, and in the specic numerical example employed to illus-V The linearly rising portion of the sweep must be long enough to permit being biased by as many as (1i-1) control currents, each delaying the coil 4 pulses by (cl--l) periods. The extent of the linearly rising portion must therefore be (iz-1) (d4-1), or n-|-nd-d-1, pulse periods. Since, as above stated, the fiat portion must be at least N periods long, there remain, therefore, at most, d-l-Z pulse periods for the falling part oi the curve, during which the transition from the linearly rising part to the flat part may take place.

Any suitable synchronizing means may be employecl in connection with the invention, a suitable one being to apply to the outgoing line at the point shown l the pulse voltage of coil .l--G itself, which may be delayed, if desired, as by a delay device I? by the interval such as to compensate for delays introduced in the coding operation and locate it in the zero pulse position of the frame as shown in Fig. 4. A suitable l characteristic may be given to this synchronizing pulse to enable the receiver terminal equipment to distinguish it from other pulses in the train. For example, it may be of greater amplitude as indicated on the figure.

At the receiver terminal, shown in Fig. 5, the incoming pulse train may be conceived of as arriving on an incoming line lil. After such ampliiication and regeneration as may be considered desirable, this pulse train is applied to the system. The synchronizing pulse may be sorted from the other pulses of the train in a manner appropriate to its distinguishing characteristics. Ii, as above suggested, it diners merely in amplitude, it may be applied by lway of an amplitude filter 'il to control the timing of a basic timing generator in well-know `fashion. Aside from the iact that it is held in synchronism with the transmitter terminal equipment in the manner just described, the timing circuit may be identical with that described above in connection with the transmitter. Thus a sawtooth voltage wave generator l2 feeds a series of delay devices 'i3-l to 'i3- 8, each of which introduces a delay of t, and from the points interconnecting which, delayed versions of the sawtooth wave may be derived'and applied to the respective grids of a group oi triodes l5 in the anode circuit of each of which is a pulse coil l similar to the coils 4 described above. The saturable core yt3 of each such coil is provided with two windings, the rst of which 'I4 is connected in the anode circuit of the tube 75, the second 'Il beingfurnished with an adjustable bias current for setting vthe quiescent value of the magnetizing force as described in connection with the transmitter apparatus. In the case of all the cores except the nrst two, a third winding 'I8 is also provided, being connected in series with the third windings 13 of the other coils of this group, all as described above in connection with the transmitter apparatus In operation, and in the absence of incoming channel pulses, the anode currents of the several tubes l5 having the wave form shown in curve A of Fig. 6 and follow one another at intervals of t. Each of the eight listeners A through H, inclusive, is furnished with a telephone instrument which is suppliedby Way of a low-pass filter from atwo-Way clamp circuit 8l. When actuated, the clamps simultaneouly discharge all of a bank of storage condensers 82 into the appropriate telephone receivers of the several listeners. This is the first event to take place in the frame cycle, and is accomplished by application of a clamping pulse from coil 'i4-9 to all of the two-way clamps at once.

The ensuing events will be described in connectionwith the example already taken in which talkers B, E and F were active, the others being idle. Thus, after the pulse from coil I4-ll has discharged the speech samples from the prior frame by way of the clamps 8l to the appropriate listeners, the next event to take place is the occurrence of a pulse in coil 'i4-I. This is applied to a time-coincidence gate III 83-l which may be of the same variety as gates I and II at the transmitter, for example, a pentode, the pulse from coil 'I4-l being applied to its control grid while the pulses of the incoming train on the line I are applied to its suppressor grid or vice versa. By hypothesis, the pulse position corresponding to the No. l channel pulse is blank so that gate III 83-1 does not operate, and the 13 pulse-from coil i4-I is thus of no. eiect. After a time t thereafter, a pulse occurs in coil 'I4-2 which is similarly applied to the gate III 83-2. At. the same instant, a channel pulse of the incoming train is applied to this gate which, like the gate 83-I,`may be a pentode. The coil 14--2 pulse may be applied to one of its grids while the pulses of the incoming train are appliedA to the other.V The coincidence of the pulses from these two sources serves to enable gate 83`2 which sends a pulse output by way of two paths. The right-hand path operates a clamp circuit 84-2 which connects a condenser 85-2 momentarily to a source of bias control voltage which may be a single trip multivibrator 86 whose output wave form is shown in curve F of Fig. 6. rThe bias control voltage, thus stored` for the remainder ofthe frame period, is applied, preferably by way of ,a cathode feedback buier 31-2 to the grid of a triode 68-2. The tube 88-2 is normally held below cut-off but when the above-men tioned clamp 84-2 operates, this tube 83-2 conducts a current from a battery 39. As inthe case of the transmitter apparatus, the resulting current owing through all of coils 'f3- 3 to IB-8, inclusive, operates to delay the occurrence of the condition in which the pulses occur at the output terminals of the remaining tubes of the groupfl, and so provides time for the complete decoding of the code pulse group which follows immediately upon the No. 2 channel pulse.

TheA decoder 9i) may be of any suitable variety, a suitable one comprising a resistor-and-condenser combination 9| having a decay factor of one-half in one pulse period, to which standard magnitude increments of charge are applied by any suitable means. The operation of such a condenser resistor combination to convert binary code pulse groups into signal amplitude samples is described in the aforementioned article published in the Bell System Technical Journal. Various means may be employed to apply the required standard increments of charge to this resistor combination. For example, a synchronously rotating cathode beam may be radially deflected to pass through an aperture 92 in a mask when, and only when, an incoming pulse arrives. Such a system is shown, for example, in an application of Frank Gray and R. W. Sears, Serial No. 785,696, led November 13,1947. With sucha decoding system, the decoded speech' sample builds up at the input: terminals of a clamp 93 throughout the duration of the code pulse group, and attains its correct value at the termination of the code pulse group, namely, after a time 'lt measured from the instant of occurrence of the No. 2 channel pulse. At'this instant the clamp 93-2 is actuated to connect a storage condenser 94-2 to the decoder 90, and thus to place on the storage condenser Sli-2 a-,total charge which is proportional to the decoded speech sample. The clamp 93-2 is operated at the correct instant by a pulse in the left-hand output path from gate III 83-2 delayed by the proper time interval, namely 715, by a suitable delay device 95-2. Immediately it has served its purpose of placing the decoded speech sample f in the form of a charge on the storage condenser 94-2, the clamp 93,-2 opens, leaving the charge on the storage condenser and isolating this condenser from further decoded speech samples which, while they are present at the input terminals of the clamp 93-2 are nevertheless destined for listeners other than B.

The next events, which take place in rapid succession, are the occurrences of pulses in coils lli- 3 and 'i4-4, separated from each other and from the recovery pulseeperiod of the channel 2 pulse group by t. This situation is illustrated in curves B and C of Fig. 6. Inasmuch as the incoming pulse train is blank at the time of occurrence of pulses from coils I4-3 and 'I4- 4, gates 83-3 and 83-4 remain disabled and nc further action takes place in the message paths for listeners C and D. The next event, after another time interval t is the occurrence of the pulse of coil 14-5. This is applied to gate III B3-5 at the same instant as the No. 5 channel pulse of the incoming train. As with the other gates 83 of this group, gate 83-5 mayv be a pentode, the coil pulse being applied to one of its grids and the incoming channel pulse to another. The time coincidence between these two pulses enables the gate which delivers an output in the right-hand direction inthe figure to the clamp Bil-5 which connects the grid of the triodes 8f3-5, by way of a buffer 81-5 to the bias control voltage source 855 which at this instant has its most positive value, thereby enablingthe triode 88-5 to conduct a current from the battery R9 by way of its discharge path, its anode resistor and the control'bias windings of coils 'IB-5, I3- andv 'F8-#8. As in the caseof channel 2, the magnitude of this control bias current is so adjusted as to effectively postpone the occurrence of the pulsing condition ineoils 'I4-'5, 'i4-'' and 'i4-48 by a time interval '715; At the same time the output of gate III i3-5 is applied by way of a delay device 95-5'which introduces .l a delay of it to the decoder clamp 93-5.A As' in the case of channel 2',the decoded speech signal will have attained its correct value'at the input terminals of the clamp at the "conclusion of the code pulse group period, namely, seven pulse periods after the No. E channel pulse. Thus the decoder clamp 93--5 is'operated at the correct time for a brief instant anda charge is placed on the storage condenser 'S4-5 whose value is proportional to the decoded signal as'it' instantaneously appears at the input terminals of the clamp. Upon removal from the clamp 93-5 of the operating pulse, the signal path controlled by this clamp isl immediately reopened, leaving the decoded 'speech' sample in the form of a charge on thev storage condenser 94-5 and isolatingk this condenser from further decoded speech samples which'would be destinedifor hearers other'than E. l 1

The next event to occur is the pulse inV coil 'I4-J5: As before, and since talker F is active, the No. t channel pulse is present intheincoming train; Gate III 83-5 is therefore opened by the time coincidence 'oi the No. 6 channel pulse andthe pulse from coil 'lli-. Opening ofi gate III S23-6 clamps the grid of triode'-i to the bias control voltage'source 86, passes a current of appropriate value from the battery 89 through this triode,v and the bias control windings 18--7 and 'iR-8 which effectively postpones the occurrence of the pulsing condition in the coils 'iii-l' and 'Fil-8 by another interval of Tt. At the same time `the output ofY gate III' S33-' Q whereupon the clamp disables the signal sample path, leaving the charge on the storage condenser and isolating it from subsequent samples which may occur in the decoder output.

By hypothesis, talkers G and H are silent, so that the last two events to take place in the frame are the occurrences of pulses in coils 'i4-I and 'I4-8. These are inefective to open the gates III 83-1 and 83-8, because at the times at which they occur, the No. I and No. 8 channel pulse positions are blank in the incoming pulse train.

At the conclusion of the frame, the original magnetic conditions of all of the cores of all of the pulse coils must be restored to their original values in preparation for operations in the ensuing frame. This is carried out, in accordance with the invention, by cutting off the discharge currents of all of the tubes 88 which reduces the currents in all of the control bias windings 18 to zero. This, in turn, is accomplished by the application of the pulse output of coil 'I4-ll of the ensuing frame, to all of the clamps 91 in parallel, thus clamping the grids of the tubes 88 to a sufficiently negative voltage, at the conclusion of each frame. The negative voltage may conveniently be derived from the bias control voltage source 86, which during the latter portion of this frame has this negative value. In particular the bias control voltage source may be a conventional single trip multi-vibrator having a relaxation time of twenty-one pulse periods, and triggered, by way of a conductor 99, into a condition in which it delivers a voltage of suitable magnitude (flat part of curve F of Fig. 6)V by the frame synchronizing pulse which arrives at the beginning of each frame. Thus, during the last few pulse positions of the frame, the voltage Voutput of this bias source has a more negative Value, and the tubes 88 are cut orf by reason of the fact that their grids are clamped, by Way of the buffers 81, to this more negative voltage under the action of the initial pulse of the ensuing frame, generated by the coil 'I4-9. This restores the magnetic conditions of all of the pulse coils 74 to their original values in which they are ready for action in the ensuing frame in the manner above described. As described earlier, the initial pulse of the ensuing frame from coil I4-0 also acts by Way of the clamps 8l to discharge all of the storage condensers 94 into the telephone receivers of the several listeners for which the speech samples stored on them are destined.

It is a feature of the particular decoder apparatus employed as a component in the present system that decoded speech samples are all of the same sign, smallest values corresponding to signal peaks in one direction, largest values corresponding to signal peaks in the other direction, and average values corresponding to portions of the speech wave which are of small or zero amplitude. Now in the case of any talker who is momentarily inactive or, if he is active, his speech sample is not transmitted because it is smaller than the threshold of the sample detector 24, the corresponding storage condenser 94 receives no charge from the decoder S0 by way of the clamp 93, and when the stored charge is discharged into the listeners telephone instrument, the resulting speech sample as applied to this telephone instrument constitutes false information, since the correct information is that which corresponds to the central part of the transmitter coding mask 50. To remedy this defect, steps must be taken to arrange Vthat zero values of the recovered speech wave correspond, at the receiver as they do at the transmitter, to average values of the charge on the storage condenser 94 during a talkspurt. Channels which are not transmitted because they are momentarily inactive must be made to contribute this average condenser charge, to avoid violent transient disturbances. vThis is accomplished by insuring that all of the storage condensers 94 shall have, at the beginning of each frame, a charge equal to the average output of the decoder 99. This result is secured by discharging each storage condenser 94 at the end of the frame not directly into the hearers telephone receiver but by way of a battery whose voltage is the average of the stored voltages.

At the end of a frame in which any particular channel has been found inactive by its sample detector 24, this correct voltage is discharged into the listeners receiver since it has been held on the storage condenser 94 throughout that frame. In the case of a channel which is active, the channel pulse acts by way of the clamp 83 and a discharge clamp IUI to discharge the particular condenser 94 to ground, thus removing all charge from that condenser prior to the instant at which the output of the decoder 90 is stored on it. In order to provide approximately equal times for charging and for discharging the storage condensers 94, each of the channel pulses, as it arrives from the associated gate III 83, is rst passed through a delay device |92 which delays it approximately Bt, and is then applied to the associated condenser discharge clamp 19|. This instantaneously grounds the condenser 94 and removes all charge from it, so that the charge increment later placed upon it by operation of the decoder clamp 93 shall be measured from zero. Thereafter, when the time arrives for discharging the condenser 94 into the listeners telephone instrument, this discharge takes place by way of the battery E00 Whose voltage is equal to the average value of the decoder output as it appears on the storage condensers 94. When thestorage condenser 94 has a charge equal to its average value, this is exactly balanced by the voltage of the battery, so that when the clamp 93 is instantaneously closed by the delayed output of the gate III 83 on the occurrence of the channel pulse, a speech sample of Zero magnitude is delivered to the listener.

The impulse coils and cores employed in the present system may be of the type described in the following publications: Magnetic Generation of a Group of Harmonics by E. Peterson, J. M. Manley and L. R. Wrathall, Bell System Technical Journal, October, 1937, volume 16, page 437; Coil Pulsers for Radar by E. Peterson, Bell 'System Technical Journal, October, 1946, volume 25, page 603; Frequency Modulation by Non- Linear Coils by L. R. Wrathall, Bell Laboratories Record, March, 1946, volume 24, page 102. The various clamps employed in the present system may be as described and shown in an article entitled An Experimental Multichannel Pulse Code Modulation System of Toll Quality by L. A. Meacham and E. Peterson, Bell System Technical Journal, January, 1948, volume 27, page 1 and especially pages 26 and 27.

The tuned speech detector may be of the type shown in Bjornson Patent 1,840,015.

What is claimed is:

1. In an elastic time division multiplex transmission system wherein speech samples of the in- 17 stantaneously active members of a group of callers are transmitted in sequence over channel facilities which are fewer in number than the number ofkmembers of the group, a number of incoming lines at a transmitter terminal, one for each caller, a number of saturable cores, one for each caller, a first winding on each core, means for applying a saw-tooth current wave to the first windings of the several cores in succession to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming linesin serial order for activity, a

second winding on the core assigned to each caller having a serial number higher than the rst, means controlled yby the activity of any caller for passing a bias control current of standard magnitude through the second windings of all cores assigned to callers having serial numbers higher than that of said active caller` in a direction to delay, by a preassigned time intere val, the derivation of the control voltages from the rst windings of all of said last-named callers, and means for transmitting the speech sample of said active caller during said time interval.

2. In an elastic time division multiplex transmission system wherein speech samples of the instantaneously active members of a group of callers are transmitted in sequence over channel facilities which are fewer in number than the number of members of the group, a number of incoming lines at a transmitter terminal, one for each caller, a number of saturable cores, one for each caller, a first winding on each core, means for applying a saw-tooth current wave to the rst windings of the several cores in suc,

cession to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming lines in serial order for activity, a second winding on the core assigned to each caller having a serial number higher than the'rst, means controlled by the activity oi any caller for passing a bias control current of standard magnitude through the second windings of all cores assigned to callers having serial numbershigher than that of said active caller,` in a direction to delay, by a preassigned time interval, the derivation of the control voltages from the first windings of all of said last-named callers, means for encoding and transmitting the speech sample of said active caller during said time interval, and means for also transmitting Within said time interval a signal indicative of the activity of said caller.

3. In an elastic time division multiplex transmission system wherein speech samples of the in- .fr

stanstaneously active members of a group of'callers are transmitted in sequence over channel facilities which are fewer in number thanY the number of members of the group, a number of incoming lines at a transmitter terminal, one for` each caller, means for generating a voltage Wave of saw-tooth form, a string of delay devices, means for applying said voltage to said string, whereby delayed replicas of said voltage appear at the terminals of the several devices in sequence, means for converting each of said delayed replicas into a current of like form, a number of saturable cores, one for each caller, a first winding on each core, means for applying one of said replica current waves to the first winding of each of said cores to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming lines in serial order for activity, a second winding on the core assigned'to eachcaller havingaserial num- 18. ber higher than the first, means controlled by the activity of any caller for passing a bias control current of standard magnitude through the second windings of all cores assigned to callers having serial numbers higher than that of said active' caller, in a direction to delay, by a preassigned time interval, the derivation of the control voltages from the rst windings of all of said `last-named callers, and means for trans mittng the speech sample `of said active caller during said time interval.

4. In an elastic time division multiplex transmission system wherein speech samples of the instantaneously active members of a group of callers are transmitted in sequence over channel facilities which are fewer in number than the number of members of the group, a number of incoming lines at a transmitter terminal, one for each caller, means for generating a voltage wave of sawtooth form, said wave comprising, in each full cycle of N+n+nd+1 pulse periods, where N is lthe total number of callers,

n is the number of active callers provided for,

d is the number of digits in the code,

a rst linearly rising portion of at least (1w-1) (d-l-l) pulse periods, a second transitional falling portion rof at most d-i-Z pulse periods, vand a third, substantially flat slightly negative portion of at least N pulse periods, a string of delay devices, means for applying said voltage to said string, whereby delayed replicas of said Voltage appear at the terminals of the several delay devices in sequence, means for converting each of said delayed replicas into a current of like form, a number of saturable cores, one for each caller, a rst Winding on each core, means for applying a saw-tooth current wave to the first windings of the several cores in succession to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming lines in serial order for activity, a second winding on the core assigned to each caller having a serial number higher than the first, means controlled Veo by the activity of any caller for passing a'bias control current of standard magnitude through the second windings of allv cores assigned to callers having serial numbershigher than that of said active caller in a direction to delay by a time interval of (d+1) pulse periods the derivation of the control voltages from the first windings of all of said last-named callers, means for converting the speech sample of said active caller into a code pulse group of d digits during said time interval, and means for transmitting, within said time interval, said code pulse group and a signal indicative of the activity of said caller.

5. In an elastic time division multiplex transmission system wherein speech samples of the instantaneously active 'mem-bers of a groupof callers at a transmitter terminal are transmitted in sequence over channel facilities which are fewer in number than the number of Amembers of the 19 ing eachV of said control voltage pulses to test said-incoming code pulse train for the presence of a channel pulse, a second winding on the core assigned to each listener having a serial number higher than the first, means controlled by the time coincidence of an incoming channel' pulse with a control voltage pulse of said series for passing a bias control current of standard magnitude through the second windings of all cores assigned to listeners having serial numbers higher than that of said active caller in a direction to delay, for a preassigned time interval, the derivation of the control voltages derived from the rst windings of all of said last-named listeners, and means for decoding the code pulse group which follows said channel pulse and converting it into a speech sample during said time interval.

6. In an elastic time division multiplex transmission system wherein speech samples of the instantaneously active members of a group of callers at a transmitter terminal are transmitted in sequence over channel facilities which are fewer in number than the number of members of the group, apparatus at a receiver station, adapted to receive successive code pulse groups destined for diierent listeners, each group being preceded by a channel pulse indicative of the instantaneous activity of a particular caller, means for generating a voltage wave of saw-tooth form, a string of delay devices, means for applying said voltage to said string, whereby delayed replicas of said voltage appear at the terminals of the several delay devices in sequence, means for converting each of said delayed replicas into a current wave of like form, a number of saturable cores, one for each listener, a rst winding on each core, means for applying one of said replica current waves to the rst winding of each of said cores in succession to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming code pulse train for the presence of a channel pulse, a second winding on the core assigned to each listener having a serial number higher than the first, means controlled by the time coincidence of an incoming channel pulse with a control voltage pulse of said series for passing a bias control current of standard magnitude through the second windings of all cores assigned to listeners having serial numbers higher than that of said active caller in a direction to delay, for a preassigned time interval, the derivation of the control voltages derived from the rst windings of all ci said lastnamed listeners, and means for decoding the code pulse group which follows said channel pulse and converting it into a speech sample during said time interval.

7. In an elastic time division multiplex transmission` system wherein speech samples of the instantaneously active members of a group of callers at a transmitter terminal are transmitted in sequence over channel facilities which are fewer in number than the number of members of the group, apparatus at a receiver station adapted to receive successive code pulse groups destined for different listeners, each such code pulse group being preceded by a channel pulse indicative of the instantaneous activity of a particular caller, a number of saturable cores, one for each listener, a rst winding on each core, means for applying a saw-tooth current wave to the rst windings of the several cores in succession to derive a series of control voltage pulses, means for utilizing each of said control voltage pulses to test said incoming code pulse train for the presence ofv a channel pulse, a second winding on the core assigned to each listener having a serial number higher than the iirst, means controlled by the time coincidence of an incoming channel pulse with a control voltage pulse of said series for passing a bias control current of standard magnitude through the second windings of all cores assigned to listeners having serial numbers higher than that of said active caller in a direction to delay, for a preassigned time interval, the derivation of the control voltages derived from the rst windings of all of said last-named listeners, means for decoding the code pulse group which follows said channel pulse and converting it into a speech sample during said time interval, a storage device and a reproducer assigned to each listener, means for storing the several speech samples so recovered on the several storage devices, and means for discharging the several storage devices individually into the several reproducers at the conclusion of each full cycle of operations.

8. In an elastic time division multiplex transmission system wherein speech samples of the instantaneously active members of a group of callers at a transmitter terminal are transmitted in sequence over channel facilities which are fewer in number than the number of members of the group, apparatus at a receiver station adapted to receive successive code pulse groups destined for different listeners, each group being preceded by a channel pulse indicative of the instantaneous activity of a particular caller, means for generating a voltage wave of sawtooth form, said wave comprising, in each full cycle of N -i-n-i-vd-l-l pulse periods, where N is the total number of callers, n is the number of active callers provided for, dis the number of digits in the code,

a first linearly rising portion of at least (n-l) (cH-1) pulse periods, a second transitional falling portion of at most d-i-Z pulse periods, and a third, substantially flat slightly negative portion of at least N pulse periods, a string of delay devices, means for applying said Voltage to said string, whereby delayed replicas of said voltage appear at the terminals of the several delay devices in sequence, means for converting each of said delayed replicas into a current of like form, a number of saturable cores, one for each listener, a rst winding on each core, means for applying a saw-tooth current wave to the iirst windings of the several cores in succession to derive a series of control voltage pulses, means for utilizing said control voltage pulses to test said incoming code pulse train for the presence of a channel pulse, a second winding on the core assigned to each listener having a serial number higher than the rst, means controlled by the time coincidence of an incoming channel pulse with a control voltage pulse of said series for passing a bias control current of standard magnitude through the second windings of all cores assigned to listeners having serial numbers higher than that of said active caller in a direction to delay, for a preassigned time interval, the derivation of the control voltages derived from the rst windings of all of said last-named listeners, and means for decoding the code pulse group which follows said channel pulse and converting it into a speech sample during said time interval.

CARL B. H. FEIDMAN.

(References on following page) 21 22 REFERENCES CITED Number Name Date u The following references are of record in the 277,192 WESOE e- Ma'r- 24" 1942 me of this ,patent-l: 2,301,223 Mltchell NOV. 10, 194:2 2,388,901 Loughlen Oct. 30, 1945 UNITED STATES PATENTS 5 2,147,233 Chatterjee Aug. 17, 194s Number Name Date 2,458,652 Sears Jan. 11, 1949 1,873,785 Ranger Aug. 23, 1932 2,473,691 Meacham June 21, 1949 1,873,786 Ranger Aug. 23, 1932 1,905,359 A1181 Apr. z5, 1933 OHER REFERENCES 2,200,559 Mitchell May 14,1949. 10 Pulse Count Modulation System, Teletech, 2,271,000y Loveu .1311271942 September 1947,1111-48-52- 

